I found something rather intriguing today on gravsim - I took the fullsystem.gsim, removed all the moons and asteroids (leaving just the planets) and increased the mass of Jupiter to 15, 50, and 70 MJ and let it run with each of those masses of Jupiter for a while at a timestep of 8192.

As the mass of Jupiter was increased, the outer system got more unstable. Even at 15 MJ, Saturn's orbit was in trouble (it wouldn't complete after even a single orbit, and started precessing). At 50 MJ, Saturn was being thrown around all over the place (getting captured a couple of times by megajupiter). At one point Saturn even got thrown into a very elliptical orbit with an aphelion just beyond Mars' orbit, but amazingly the inner planets survived the experience with their orbits about the same as they were before! After that it ended up captured by megaJupiter again and then thrown out of the system completely. And at 70 MJ, Saturn's first orbit after the change wasn't even elliptical anymore, it was being distorted so much!

The interesting thing is that as megajupiter's mass increased, Uranus and Neptune's orbit became more polygonal (Uranus' was 8-sided, Neptune's was 16-sided). I wonder why that would be - some kind of resonance? Also as megajupiter's mass increased their orbits became less stable too.

I wonder why the inner planets didn't appear to be affected at all by any of this though? Their orbits seemed stable throughout. Are they just close enough to the sun that the sun's gravity dominates and keeps their orbits stable? Looking at them close-up, Mars' orbit seems to be precessing very slightly, but the orbits of the rest of the inner planets are unchanged.

The attached view shows what the solar system looks like in the year 2200, having increased the mass of Jupiter to 50 MJ in 2005 (so this is after only about 200 years of evolution). You can see the polygonal shapes of Uranus' and Neptune's orbits, and Saturn's being thrown around all over the place (I guess due to its 5:2 orbital resonance with jupiter?).

The strange shapes are probably caused because you have the Sun locked to the middle of the screen. With Jupiter that massive, the barycenter of the solar system may no longer be approximated as the Sun. On the "Graphics Options" interface, press the "F" and "A" button to lock/unlock the Sun.

The inner planets are very close to the Sun. As far a megaJupiter is concerned, they are part of the Sun. Their orbital velocities are very large compared to Saturn, Uranus, and Neptune, so a perturbation of a few km/s won't affect them as much as their orbital velocities are measured in 10s of km/s, rather than <10 as is the case for the outer planets.